AP BIOLOGY

Detailed explanation-1: -Cyclins are proteins that regulate the timing of the cell cycle in eukaryotic cells. Regulatory proteins are proteins that control the cell cycle both inside and outside of the cell. Internal regulators allow the cell cycle to proceed when certain events have occurred INSIDE a cell.

Detailed explanation-2: -Cell cycle progression is regulated in part by the sequential activity of various cyclins. The cyclins are regulatory subunits that bind, activate and provide substrate specificity for their catalytic partner serine-threonine kinases, collectively called cyclin-dependent kinases (Cdks) (reviewed in refs.

Detailed explanation-3: -Internal regulators of the cell cycle consist of two formal, well-defined types: positive regulator molecules such as cyclins and cyclin-dependent kinases and negative regulator molecules such as Rb, p53 and p21.

Detailed explanation-4: -The progression of cells through the division cycle is regulated by extracellular signals from the environment, as well as by internal signals that monitor and coordinate the various processes that take place during different cell cycle phases.